1. Field of the Invention
The present invention relates to a radio apparatus and an antenna device, and in particular to an antenna device including magnetic material for isolation and a radio apparatus having the antenna device.
2. Description of the Related Art
A small-sized radio apparatus such as a mobile phone often has limited mounting space, and thus, in some cases, suffers from a problem of interference caused by electromagnetic or capacitive coupling between an antenna and each of portions of an electrical circuit of the radio apparatus. In particular, in some cases, the antenna suffers from a problem of degraded radiation efficiency caused by coupling with a conductive portion of a circuit board or a housing (called a peripheral conductive portion hereafter).
To the above problems, a solution by means of magnetic material for isolating an antenna from a peripheral conductive portion has been studied. As an antenna adopting such a solution, an antenna module adapted for a card of a radio frequency identification (RFID) system is disclosed in Japanese Patent Publication of Unexamined Applications (Kokai), No. 2005-80023.
The antenna module of JP 2005-80023 is constituted by an antenna board provided with an antenna coil, a magnetic core material and an interference shielding plate layered on top of each other. The magnetic core material of the above antenna module shows different magnetic characteristics between on a face on a side of the antenna coil and on a face on a side of the interference shielding plate, so as to cope with both a communication characteristic of the antenna coil and an interference shielding effect.
Furthermore, a technology for interposing a sheet member including magnetic material between an antenna element and a conductive material so as to increase radiation efficiency is disclosed, e.g., in Japanese Patent Publication of Unexamined Applications (Kokai), No. 2007-124638. The sheet member of JP 2007-124638 is formed by an interference shielding layer formed by the magnetic material, a conductive layer and an adhesive layer, and is arranged to prevent antenna impedance from decreasing by selecting a magnetic permeability value of the interference shielding layer, and so on.
According to JP 2005-80023 described above, a filling ratio of soft magnetic powder is relatively lowered and the insulation characteristic is enhanced on the face of the magnetic core material on the side of the antenna coil, so that an eddy current is prevented from occurring and the loss of the antenna coil is reduced. The filling ratio of the soft magnetic powder is relatively raised on the face of the magnetic core material on the side of the interference shielding plate so that electromagnetic isolation is reinforced between the antenna board and the interference shielding plate.
If an antenna and a peripheral conductive portion are electro-magnetically isolated by means of magnetic material, it is important to reduce an eddy current and to reinforce electromagnetic isolation in parallel. The antenna module of JP 2005-80023 has the magnetic core material formed by a plurality of layers of different filling ratios of the soft magnetic powder so as to meet both the above requirements. The antenna module of JP 2005-80023 has a problem, however, in that it requires a manufacturing process for selecting a plurality of kinds of material of different characteristics and layering them on top of each other.
The technology disclosed in JP 2007-124638 uses a method such as selecting a mixing ratio of a plurality of kinds of soft magnetic powder. The configuration of JP 2007-124638 has a problem in that it requires a manufacturing process for selecting such material and layering them similarly as the configuration of JP 2005-80023.